The present invention relates generally to systems and methods for performing noninvasive procedures using focused ultrasound, and more particularly to systems and methods for controlling or steering a multiple element focused ultrasound transducer array.
High intensity focused acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors, thereby obviating the need for invasive surgery. For this purpose, piezoelectric transducers have been suggested that may be placed external to the patient but in generally close proximity to the tissue to be ablated and driven by electric signals to produce ultrasonic energy. The transducer is geometrically shaped and positioned such that the ultrasonic energy is focused at a xe2x80x9cfocal zonexe2x80x9d) corresponding to a target tissue region within the patient, heating the target tissue region until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another. This series of sonications may be used to cause coagulation necrosis of an entire tissue structure, such as a tumor, of a desired size and shape.
A spherical cap transducer array has been suggested for this purpose that includes a plurality of concentric rings disposed on a curved surface having a radius of curvature defining a portion of a sphere. The concentric rings may be divided circumferentially into a plurality of curved transducer elements or sectors, creating a tiling of the transducer face. The transducer elements are driven by radio frequency (RF) electrical signals at a single frequency offset in phase and amplitude. In particular, the phase and amplitude of the respective drive signals may be controlled so as to focus the emitted ultrasonic energy at a desired xe2x80x9cfocal distance,xe2x80x9d i.e., the distance from the transducer to the center of the focal zone and provide a desired energy level in the target tissue region.
In addition, the phase of the respective drive signals to each of the sectors may be controlled to create a desired size and shape for the focal zone. For example, if each of the sectors are driven with respective drive signals that are in phase with one another (xe2x80x9cmode 0xe2x80x9d), the ultrasonic energy may be focused substantially at a relatively narrow focal zone. Alternatively, the sectors may be driven with respective drive signals that are in a predetermined phase relationship with one another (referred to as xe2x80x9cmode nxe2x80x9d). This results in a focal zone generally defining an annular shape, creating a wider focus that causes necrosis of a larger tissue region within a focal plane intersecting the focal zone.
Directing acoustic energy through irregular tissue structures, such as bone, may further complicate focusing the acoustic energy at a desired tissue region to be treated. For example, when treating tissue within a skull, the non-uniform interior surface of the skull may cause irregular phase shifts in acoustic energy as it passes through different regions of the skull. It may be difficult to compensate for these irregularities, requiring special calibration of the transducer.
Even if the transducer is calibrated to compensate for aberrations caused by such irregularities, the focus may need to be moved during treatment to focus the acoustic energy at one or more target tissue regions. One option for moving the focal zone is to physically move the transducer with respect to the anatomical structure. Such xe2x80x9cmechanicalxe2x80x9d steering may be used, for example, to control a hemispherical transducer for treating the skull. However, this requires that the transducer be substantially larger than the skull, e.g., about thirty centimeters (30 cm) or more in diameter, in order to provide sufficient freedom of movement. In addition, the space between the transducer and the skull must be substantially filled with a fluid to acoustically couple the transducer and the skull, which may complicate such a steering system.
Alternatively, electronic steering may be used, similar to that described above, in which the relative phase of the acoustic energy emitted by transducer elements making up the transducer is controlled to move the focal zone. The degree of control provided by such electronic steering is inversely proportional to the size of the individual transducer elements. For example, it is generally desirable to have the size of the transducer elements be on the order of the wavelength of the acoustic energy emitted by the array, and preferably as small as half the wavelength, in order to effectively steer the focal zone. Thus, with acoustic energy having a wavelength on the order of two millimeters (2 mm), as is often used for focused ultrasound systems, transducer elements having a similar size, i.e., about two millimeters or less in cross-section would be needed for effective steering.
For an exemplary transducer array having a radius of six centimeters (6 cm) and a desired focal distance of about twelve centimeters (12 cm), this would require approximately three thousand (3,000) elements. Even more dramatically, for an array formed as half of a sphere sized to fit around a skull, this may require as many as about twelve thousand (12,000) elements, which would be prohibitively expensive to make and complicated to operate.
Accordingly, it is desirable to provide systems and methods for treating a tissue region using a multiple element focused ultrasound array that facilitate improved control and/or focusing of the elements in the transducer array.
The present invention is directed to systems and methods for performing a diagnostic or therapeutic procedure using focused ultrasound, and more particularly to systems and methods for mechanically steering or otherwise controlling a multiple element transducer array to facilitate focusing of the transducer array at one or more regions during a therapeutic ultrasound procedure.
In accordance with one aspect of the present invention, a system is provided that includes a transducer array including a plurality of sub-arrays, each sub-array defining an acoustic emission surface and including one or more, and preferably a plurality of, transducer elements configured for emitting acoustic energy from the acoustic emission surface, each sub-array being independently rotatable with respect to one another. In a preferred embodiment, each sub-array includes a gimbal apparatus pivotably fixing the sub-arrays with respect to one another, for example, to a common support structure.
Drive circuitry is coupled to the transducer elements, the drive circuitry configured for providing respective drive signals to the transducer elements, whereby the transducer elements may emit acoustic energy from their respective acoustic emission surfaces. An electronic controller may be coupled to the drive circuitry, the electronic controller configured for controlling phase shift values and amplitudes of the respective drive signals to further focus the acoustic energy emitted by the transducer elements towards the target region. For example, the electronic controller may be configured for controlling phase shift values of the drive signals to the transducer elements of a respective sub-array for controlling a focal distance of the acoustic energy emitted by the transducer elements in the respective sub-array. Preferably, the electronic controller controls phase shift values of the drive signals between respective transducer elements of each sub-array and/or between respective sub-arrays, e.g., to control a focal distance of the focal zone generated by the transducer array.
A mechanical controller is coupled to the sub-arrays that is configured for moving the sub-arrays to adjust an orientation of the acoustic emission surfaces of respective sub-arrays to facilitate focusing of the acoustic energy emitted by the transducer elements towards the target region. In a preferred embodiment, the mechanical controller includes an actuator coupled to each of the sub-arrays for mechanically steering the respective sub-array to adjust its orientation. More preferably, the mechanical controller may include a pair of actuators coupled to each sub-array, the pair of actuators configured for pivoting the respective sub-array in substantially orthogonal directions.
The transducer array may be mounted within a fluid-filled casing, for example, having a substantially concave shape. In a preferred embodiment, the casing has a substantially concave inner contact surface configured for substantially engaging a skull of a patient. The sub-arrays are preferably arranged within the casing such that the acoustic emission surfaces of the sub-arrays are generally oriented towards the inner contact surface.
The system may be used to perform a therapeutic procedure, such as an ablation procedure, in a target tissue region. Alternatively, the system may be used to perform a therapeutic procedure, e.g., involving acoustic energy of lower intensity than that generally used to ablate tissue, or a diagnostic procedure, e.g., ultrasound imaging.
The transducer elements may be driven with respective drive signals such that the transducer elements emit acoustic energy towards a target region. Prior to or while activating the transducer elements, the sub-arrays may be steered with respect to one another to focus the acoustic energy generated by the transducer elements towards the target region. For example, the sub-arrays may be pivoted about respective fixed points to adjust an angular orientation of their respective acoustic emission surfaces. Preferably, the sub-arrays are pivoted about two respective axes of rotation that are substantially perpendicular to one another.
In a preferred method, the transducer array is acoustically coupled to a skull, for example, by disposing the transducer array around the skull, such that the target region is a tissue structure, such as a tumor, within the skull. The transducer elements are driven with respective drive signals such that the acoustic energy emitted by the transducer element substantially ablates the tissue structure. Phase shift values of drive signals driving the transducer elements of each sub-array may be controlled such that the acoustic energy emitted by the respective sub-array is focused at a predetermined focal distance or is focused at a focal zone having a predetermined shape.